1. Field of the Invention
This invention relates generally to positioning systems, and more particularly, to systems and methods for verifying consistent measurements in un-synchronized GPS positioning.
2. Related Art
The Global Positioning System (GPS) is an example of a Satellite Positioning System (SATPS), which is maintained by the U.S. Government. GPS is satellite-based using a network of at least 24 satellites orbiting 11,000 nautical miles above the Earth, in six evenly distributed orbits. Each GPS satellite orbits the Earth every twelve hours.
One function of the GPS satellites is to serve as a clock. Each GPS satellite derives its signals from an on board 10.23 MHz Cesium atomic clock. Each GPS satellite transmits a spread spectrum signal with its own individual pseudo noise (PN) code. By transmitting several signals over the same spectrum using distinctly different PN coding sequences the GPS satellites share the same bandwidth without interfering with each other. The code is 1023 bits long and is sent at a rate of 1.023 megabits per second, yielding a time mark, sometimes called a “chip” approximately once every micro-second. The sequence repeats once every millisecond and is called the coarse acquisition code (C/A code.) Every 20th cycle the code can change phase and is used to encode a 1500 bit long message, which contains “almanac” data for the other GPS satellites.
There are 32 PN codes designated by the GPS authority. Twenty-four of the PN codes belong to current GPS satellites in orbit and the 25th PN code is designated as not being assigned to any GPS satellite. The remaining PN codes are spare codes that may be used in new GPS satellites to replace old or failing units. A GPS receiver, using the different PN sequences, searches the signal spectrum looking for a match. If the GPS receiver finds a match, then it has identified the GPS satellite, which generated that signal.
Ground based GPS receivers use a variant of radio range measurement methodology, called tri-lateration, to determine the position of the ground based GPS receiver. The tri-lateration method depends on the GPS receiving unit obtaining a time signal from the GPS satellites. By knowing the actual time and comparing it to the time that is received from the GPS satellites that receiver can calculate the distance to the GPS satellite. If, for example, the GPS satellite is 12,000 miles from the receiver, then the receiver must be located somewhere on the location sphere defined by a radius of 12,000 miles from that GPS satellite. If the GPS receiver then ascertains the position of a second GPS satellite it can calculate the receiver's location based on a location sphere around the second GPS satellite. The two spheres intersect and form a circle with the GPS receiver being located somewhere within that location circle. By ascertaining the distance to a third GPS satellite the GPS receiver can project a location sphere around the third GPS satellite. The third GPS satellite's location sphere will then intersect the location circle produced by the intersection of the location spheres of the first two GPS satellites at just two points. By determining the location sphere of one more GPS satellite whose location sphere will intersect one of the two possible location points, the precise position of the GPS receiver is determined to be the location point located on the Earth. The fourth GPS satellite is also used to resolve the clock error in the receiver. As a consequence, the exact time can also be determined, because there is only one common time offset that accounts for the position calculations of all the GPS satellites. The tri-lateration method may yield positional accuracy on the order of 30 meters; however the accuracy of GPS position determination may be degraded due to signal strength and multi-path reflections.
As many as 11 GPS satellites may be received by a GPS receiver at one time. In certain environments such as a canyon, some GPS satellites may be blocked out, and the GPS position determining system may depend for position information on GPS satellites that have weaker signal strengths, such as GPS satellites near the horizon. In other cases overhead foliage may reduce the signal strength that is received by the GPS receiver unit.
Recently mobile communication devices such as cellular telephones, or mobile handsets, have been incorporating GPS receiver technology using multiple dedicated semiconductor chips to implement a communication portion and other dedicated semiconductor chips to implement a GPS sub-system of the mobile communication device. Such mobile handsets operate in connection with a mobile communications network for telecommunications services, and in connection with the GPS system to obtain the position of the mobile handset. In mobile handsets with integrated GPS receivers, time information obtained from the mobile communications network may be provided to the GPS receiver in order to reduce the search space for detecting satellites. Systems that make time information from a mobile communications network available to the GPS receiver are known generally as assisted GPS systems (A-GPS). In A-GPS systems, coarse or precise user position and time information can be readily provided from the network to help reduce TTFF (time to first fix)—an important GPS performance parameter.
In situations of autonomous operation or precise aiding, where GPS time is either decoded, or position and time are precisely assisted, all the generated measurements are frame-synchronized without millisecond ambiguity in transmit time. Thus, synchronized positioning is applied. However, it may require a substantial amount of time to obtain frame synchronization when the signal strength is low or when the dynamics is high. The frame synchronization may even fail for various reasons. Some applications (e.g. E911) may not be able to wait such a long time for the acquired satellites to be frame synchronized.
The problem can be solved when the receiver operates in assisted mode in A-GPS system, which provides measurements and computes positioning even before frame or bit synchronization. The methodology is called un-synchronized positioning, which works under the condition that all the un-synchronized measurements can be made consistent without millisecond ambiguity between satellites based on the priori user position and time information.
The receiver typically checks the ranging difference uncertainty between the satellites, based on the priori position and time uncertainty of the receiver, to determine if the adjusted measurements can be made consistent or not in order to apply un-synchronized positioning. The consistency condition is only met when the user position and time uncertainty is under a certain limit, for example a safe limit of 75 km for position uncertainty in some applications assuming time uncertainty has far less effect on the ranging difference.
However, when the limit is exceeded, the measurements may not be made consistent and the resulting positioning information can be totally wrong. When this happens, the GPS receiver may have to obtain frame synchronization for a minimum number of four satellites before the valid positioning can be provided, which in return slows down the acquisition of the rest visible satellites especially when the signal strength is weak. This is not tolerable in applications in urgent needs of positioning such as E911. Therefore, criterion is needed to check if the measurements can be made consistent or not when the uncertainty combination of user position and time is greater than 75 km.
In view of the above, there is a need for methods and systems that allow GPS receivers to tolerate larger position and time uncertainties and improved time-to-first-fix (TTFF).